I am stumped. Have a coil that locks on immediately at power up. Solenoid 05A, lower right eject hole. If I disconnect J11 on the MPU the coil does not energize, so I assumed the problem was on the MPU, not a wiring issue. I replaced the TIP122 and predrive transistor 2N4401 associated with that solenoid (even though they tested good on the bench). I tested the associated diodes for that solenoid on the auxillary driver board and they test good (with J4 the auxiliary driver board disconnected). I tested U19 on the MPU per the wiki test instructions and it appears good. What am I missing? Should I change U19 or is there something else I should be testing?

deleted....doing more testing.

Game boots and starts ok, but several solenoids are not working. 1A, 2A, 4A, and 6A. Some of the A side coils are working, so I am ruling out the A/C relay.

I haven't found any shorts.

I used a DMM to test U17, 18, 19, and U20. I also used a logic probe but didn't make sense of the results.
I'm finding all sorts of things not working right. Still probing and searching for help. Now I found a coil that is getting hot after a minute, but it ohms out good. Could this be a bad TIP?

Found the A/C relay on driver board with burnt contact and warped leaf. Going to replace it. That may only be part of the problem.

IMG_4046 (resized).JPG

Replaced the bad relay. Still have several coils lock on (knocker and lower right eject hole) when there is no ball in outhole. If I put 2 balls in the trough (past the outhole), then the outhole locks on too. Weird.

I have replaced the TIP122's and predrivers associated with the coils that lock on. No change.

If I remove J11 on the MPU, then the coils do not energize (lock on).

I replaced the 7408 associated with the locked on coils. No change.

Is my next step to replace U28 or the PIA?

Solved. Replaced U28.